Isocyanates are important chemicals. For example, the world production of isocyanates exceeded 5 megatons in 2001. Traditionally, isocyanates are manufactured on a commercial scale by reaction of phosgene with amines or amine salts. The reaction, however, has several serious drawbacks. Phosgene is an extremely toxic reagent and a stoichiometric amount of HCl is produced as a by-product. Furthermore, HCl causes serious corrosion, and a stoichiometric amount of NaOH is required to neutralize the HCl, where the same amount of NaCl is formed. As restrictions upon the use of very toxic materials such as phosgene within the chemical industry have become more rigorously enforced, there has been increasing interest in developing alternative methods to phosgene in the synthesis of isocyanate.
As an alternative, the use of dimethyl carbonate or dimethyl sulfate as phosgene substitutes are relatively expensive for commercial applications (see M. Selva et. al., Tetrahedron Letters. 2002, 43 (7), 1217-1219; JP 20044262835; WO 98/56758; WO 99/47493).
Many others strategies for non-phosgene routes, including reductive carbonylation and oxidative carbonylation by using CO as carbonyl source, have been reported. One promising alternative approach that has been the subject of research in recent years involves the oxidative carbonylation of amines to carbamates in the presence of an alcohol, usually methanol, followed by catalytic decomposition of the carbamates to isocyanates.
Alper and Hartstock (J. Chem. Soc., Chem. Commun. 1141, 1985) disclose catalytic systems including palladium chloride, copper chloride and hydrochloric acid to produce carbamates from amines. This Wacker-type catalytic system, consisting of PdCl2—CuCl2—HCl, is disclosed as being effective at mild conditions (1 atm and room temperature) in the oxidative carbonylation of amines to produce high yields of carbamate. In this system carbon monoxide (CO) and oxygen (O2) are bubbled through an alcohol to which is added PdCl2 and, finally, the amine. The mixture is stirred overnight, at room temperature and pressure, and filtered. The filtrate is subject to rotary evaporation. The resulting oil is treated with either diethyl ether or acetone and filtered, and concentration of the filtrate yields the carbamate ester. Further purification is carried out by thin-layer or column chromatography (silica gel).
Gupte and Chaudhari, Journal of Catalysis, 114, 246-258, 1988, studied the oxidative carbonylation of amines using a Pd/C—NaI catalytic system. Although effective at producing carbamates, this catalytic system uses a CO/O2 molar ratio of 5/1, which is inside the flammability envelope.
US 2002/0183541 employ a Group VIII metal catalyst and/or copper-based catalyst with halide promoters to produce carbamate esters through heterogeneous oxidative carbonylation in a gas-solid carbonylation process. The carbamate produced remains on the catalyst surface and must be recovered through expensive extraction and distillation steps.
T. W. Leung, J. Chem. Soc. Chem. Comm., 3, 1992, 205-6 and U.S. Pat. No. 5,194,660 describes a process for producing carbamates, using a homogeneous catalyst that comprises contacting a first reactant selected from primary amine components, secondary amine components, urea components and mixtures thereof; carbon monoxide; at least one oxygen-containing oxidizing agent, in the presence of catalyst composition comprising at least one metal macro-cyclic complex, preferably in the further presence of one iodine component. The macro-cyclic complex is selected from the group consisting of metal porphyrin or metal phthalocyanine including a metal selected from the metals of group IIIa to Va and group VIII of the Periodic Table and at least one iodine component is present in an amount effective to facilitate the formation of the carbamate.
A. Bassoli et al., J. Mol. Catal. 1990, 60, 41 teaches the formation of ureas in good yields, with small amounts of carbamates and azo derivatives via the N,N-bis(salicylidene)ethylenediaminocobalt(II)-catalyzed oxidative carbonylation of aromatic primary amines in methanol.
E. Bolzacchini et al., J. Mol. Catal. A: Chemical, 111, 1996, 281-287, describes the N,N-bis(salicylidene)ethylenediaminocobalt(II)-catalyzed oxidative carbonylation of substituted aromatic primary amines in methanol to give blends of ureas, isocyanates, carbamates and azoderivatives. Such blends are unsuitable for the synthesis of commercial isocyanates. Further, the long reaction times required (48 hours) precludes the practical industrial application of this approach.
US Patent Application Publication 2003/0162995 describes a one-pot synthesis of isocyanates by reaction of amines with dimethyl carbonate and subsequent heating to obtain the isocyanate. Due to reaction conditions, the separation of the isocyanate product involves a complicated separation process which comprises water addition, further heating, filtration and a number of distillations in order to obtain the isocyanate, in impure form, which then must be further purified.
Therefore, there is an extensive literature regarding the production of isocyanates and polyisocyanates. The most commonly used procedure involves the transformation of an amine into the corresponding carbamate in a first step, followed by the thermal decomposition of the carbamate to obtain the desired isocyanate or polyisocyanate. A large number of prior publications refer to one of these two steps.
For example, as mentioned above, U.S. Pat. No. 5,194,660 discloses a method for the preparation of carbamates. However, the synthesis of the corresponding isocyanates is only suggested and only from the corresponding carbamates after isolation. Further, according to U.S. Pat. No. 5,194,660 it is necessary to isolate the carbamate intermediate prior to its conversion into the desired isocyanate.
Only a few references mention or suggest the possibility of direct transformation of amines into the corresponding isocyanates. However, as in US Patent Application Publication 2003/0162995, they usually require complicated separation steps.
In view of all of the above, there is an existing need to provide an alternative cost-effective and efficient method for the direct synthesis of isocyanates and polyisocyanates.